1. Field of the Invention
The present invention relates to a method and a program for calculating a correction value for correcting a geometric error in a machine tool having translational axes and rotational axes.
2. Description of Related Art
FIG. 1 is a schematic view of a machine tool (a five-axis control machining center or a five-axis machine) having three translational axes and two rotational axes, as an example of the above-described machine tool. A spindle head 2 can make motions with two degrees of freedom of translation with respect to a bed 1, along X- and Z-axes as translational axes, which are perpendicular to each other. A table 3 can make motions with one degree of freedom of rotation with respect to a cradle 4, around a C-axis as a rotational axis. The cradle 4 can make motions with one degree of freedom of rotation with respect to a trunnion 5, around an A-axis as a rotational axis, and the A-axis and the C-axis are perpendicular to each other. The trunnion 5 can make motions with one degree of freedom of translation with respect to the bed 1, along a Y-axis as a translational axis, which is perpendicular to the X- and Z-axes. The respective axes are driven by a servomotor (not shown) that is controlled by a numerical control device, an object to be machined (a work) is fixed on the table 3, and the spindle head 2 is fitted with a tool and rotated to machine the object to be machined while controlling the position thereof relative to the tool.
Examples of factors having an influence on the accuracy in the motion of the five-axis machine include geometrical errors (geometric errors) among the respective axes. For instance, an error in the center position of each of the rotational axes (a deviation from a prescribed position), an error in the gradient of each of the rotational axes (a degree of perpendicularity or parallelity between the axes), and the like. The presence of a geometric error leads to a deterioration in the accuracy in the motion of the machine tool, and to a deterioration in the accuracy in the machining of the object to be machined. Therefore, the geometric error needs to be reduced through adjustment, but it is difficult to completely eliminate the geometric error. The machining can be carried out with high accuracy by performing a control of correcting the geometric error.
A method as described in Japanese Patent Application Publication No. 2004-272887 (JP 2004-272887 A) has been proposed as means for correcting a geometric error. In the method described in that, an error in the position of a distal end of a tool resulting from a geometric error in a machine tool can be corrected by converting the position of the distal end of the tool into positions on respective translational axes in consideration of the geometric error, and designating those positions as command positions. On the other hand, according to Japanese Patent Application Publication No. 2009-104317 (JP 2009-104317 A), an error in the position of a distal end of a tool resulting from a geometric error can be corrected by performing a control in which a value representing a difference between a position of the distal end of the tool with respect to an object to be machined in the case where there is a geometric error and a position of the distal end of the tool with respect to the object to be machined in the case where there is no geometric error is adopted as a correction value for a translational axis.
In these methods, in the case where an error in the gradient of a rotational axis is corrected, a command to correct a translational axis is issued as the translational axis operates. Therefore, even when only one of the translational axes is operated, the other translational axes operate in an infinitesimal manner. For example, in the case where there is an error in the degree of parallelity between the X-axis and the A-axis, even when only the X-axis is operated, the Y-axis or the Z-axis operates in an infinitesimal manner.
Such an operation may adversely affect the accuracy in machining such as flattening, drilling, or the like. For example, in the five-axis machine of FIG. 1, in the case where the A-axis is inclined with respect to the X-axis by an angle β due to a geometric error in the rotation around the Y-axis as shown in FIG. 2, it is assumed that flattening is carried out by a square end mill (a tool) 6, with a feed direction coincident with a direction from the obverse to the reverse side of the sheet of FIG. 2, and with a pick direction coincident with a direction indicated by a thick arrow P. Then, the distal end of the tool is positioned in the pick direction on a point group on a line inclined with respect to the X-axis by the angle β, due to a correction. That is, respective positioning positions Q in the pick direction are arranged on the line inclined by the angle β, so that a step is produced on a surface to be machined. Further, in the case where a rectilinear axis is guided through sliding, when this rectilinear axis is operated in an infinitesimal manner as described above, the axis moves sometimes, and does not move at other times. That is, so-called “stop feed” occurs to bring about a deterioration in the properties of a machined surface, such as the creation of irregularities along the machined surface or the like. In addition, in the case where drilling is carried out by a drill instead of flattening by the square end mill 6, the Z-axis is fed in a direction that forms the angle β with respect to the Z-axis, which is the axial direction of the drill. Therefore, an abnormality in the diameter of a bore occurs, and also a reduction in the life span of the drill are incurred.